Production of carbon dioxide



Patented Dec. 16, 1952 UNITED STATES PATENT QFFIfCEi PRODUCTION OF CARBON BIOXIBE' Warren W. Miller, State College, Pa., John Turkevich, Princeton, N. J., and Norman Zwiebei, Whitestone, N. Y., assignors to the United States of America as represented by the United. States Atomic Energy Commission No Drawing. Application June5, 1951, Serial, No. 230,066

6 Claims. 1

This invention relates to the productionfoi carbon dioxide and more particularly it isv concerned with a method of releasing carbon dioxide, from a thermally stable carbonate salt.

This application is a continuation-impart of our co-pending application, Serial Number 72,546, filed January 24, 1949, now abandoned.

The radioactive carbon isotope, carbonel, is an extremely valuable tracer element in organic chemical processes and in many biological processes, particularly because of its long half-life. In order to manufacture carbon-14, a thermally stable nitrogenous compound is irradiated with neutrons, in a nuclear reactor. At least a portion of the nitrogen is converted to carbon-l4 by the absorption of a neutron and emission of a proton. The resulting compound is oxidized to yield carbon dioxide which is removed from the compound and absorbed in a metal hydroxide such as barium hydroxide to yield the. corresponding carbonate salt. It is then necessary. to release the radioactive carbon dioxide from the carbonate salt. It is desired particularly to obtain the carbon dioxide. in a pure dry state and also to release substantially all of the valuable carbon dioxide from the carbonate salt.

Theusual method of accomplishing this has been to treat the barium carbonate in vacuo with a concentrated dehydrating acid, such as phosphoric. or sulphuric acid. The principal difficulty with this method is that a crust of insoluble sulphate or phosphate salt forms around the particles of the carbonate salt and thus inhibits complete recovery of the carbon dioxide. Other acids may be used but they produce water in the reaction which is vaporized and mixes with the fcarbonv dioxide gas. Even solid acidic salts, such as potassium bisulphate, are not useful for this purpose since they too produce Water in the reaction. Inorganic acid anhydrides, such as B203, may be used in the molten statebut they require so high a temperature that their use is not feasible. On the other hand, most organic anhydrides either sublime or distil under the high vacuum which is used in manufacturing the carbon dioxide from the carbonate salt. Calcination reactions are seldom feasible because of :the highv temperatures required to decompose suchcarbonates as barium carbonate. Furthermore, additants which react with air or decompose to produce acidic gases and thereby tend to reduce the calcination temperature cannotvbe used because there is no air present at the high vacuum employed and. there is the ever-present 2 problem of maintainingthe carbon dioxidefree of contaminants.

Accordingly, one of the objects ofthis invention is to provide an improved methodof decom.-- posing a thermally stable metal carbonate. An? other of the objects of this invention is top'ro-i vide a method of decomposingarelatively-thermally stable metal carbonate at a relatively low.

temperature to obtain pure, dry carbon dioxide free from any solvent. A further objectis to provide a method of preparing pure, dry-carbon dioxide.

stable metal carbonate ata low temperature, the method'to be carried out in the absencev ofv air, moisture and solvent. become apparentin the following description.

In accordance with the present invention, the carbon dioxide is obtained from the thermally stable carbonate salt in a dry and pure state by heating the carbonate in a highvacuum in the invention lead chloride (melting point 501 C.) isreacted with barium carbonate at an elevated.

temperature at a pressure preferably oflabout 10- mm. The temperature at which thereaction is carried out will depend primarily upon. the nature of this metal salt. For example, with lead chloride a measurable quantity of carbon.

dioxide is released at a temperature of about; 180 C. It is preferred, however, to. employ" temperatures within C. of the meltingpointof the metal salt so that the evolution-of the carbon dioxide from the carbonate. salt hat apractical rate. It is desirablealso to employa.

large excess of the metal salt in order toshorten the time required for substantially complete reaction. This excess may be of the order of 250 to 500% of the molar quantity requiredin the-- reaction. The pressure is, not particularlycritie cal so long as. a high vacuum is maintained- However, it is most desirable to useas low a pressure as possible, for example 10* mm.

The following examples are presented .to 1111157.

trate the method of the present invention with.

A still further objectis to provide, a method of decomposing completely a thermallyv Still other. objects will The anhydrous, salt the understanding that the invention should not be construed as being limited by the details contained therein.

Example 1 An intimate mixture was prepared containing 53 grams of pure, dry, powdered anhydrous lead chloride and 9.67 grams of pure, dry, powdered barium carbonate. The mixture was placed in a sealed container and the container evacuated overnight to a pressure of the order of 10 millimeters of mercury absolute. The mixture was then heated in the evacuated container and the rate of increase in the pressure observed as a measure of the rate of reaction, the container having a volume of about 1610 cubic centimeters. Between 240 and 250 C. the pressure increased at the rate of about 1 millimeter of mercury per minute. After a relatively short time this temperature was raised to 300 C. and the rate of increase in pressure again measured. For minutes at this temperature the rate of increase was approximately 5 millimeters per minute and after minutes the rate had fallen to 3 millimeters per minute. The heating was continued and after 40 minutes the rate had fallen to approximately 2.2 millimeters per minute while after 3 hours the rate of increase of pressure had fallen to 0.5 millimeter per minute.

After 3 hours the temperature was raised to 335 C. and at this temperature the rate of increase of pressure was approximately 1 millimeter per minute. The rate fell to about 0.7 after minutes. On increasing the temperature to 367 C. the rate was increased to 1.3 millimeters per minute where it remained for about 10 minutes. After 15 minutes at 367 (3., however, the rate fell to about 1 millimeter per minute.

Example 2 When the dry barium carbonate and lead chloride in about the same proportions as Example 1 were heated together under the same high-vacuum conditions at a temperature of about 400 C., the evolution of carbon dioxide was about 65% complete within 1 hour, the temperature having been raised to 400 C. in less than minutes. After 2 hours about 80% of the carbon dioxide had been evolved and after 4 hours the reaction was 96% complete.

In carrying out this reaction the metal salt may be fused if desired and when this is done the reaction proceeds at a greater rate both as a result of the increased temperature and of the greater mobility of the fused salt which We believe reacts directly with the stable carbonate. It may often be desirable to employ a particular fused salt at a temperature below the melting point of the salt. In such an instance a molten mixture of salts may be used such as silver chloride with lead chloride. The lowest temperature is obtained by employing the eutectic mixture. Thus when the silver chloride-lead chloride eutectic is employed the reaction proceeds at least to 98% of completion in less than an hour at 310 0., the melting point of the mixture.

Salts other than lead chloride may also be used. For example, silver chloride may be used alone as may lead iodide or other metal halide salts, such as beryllium chloride, cadmium iodide, copper bromide, etc. which are readily obtained in the anhydrous state. Similarly, anhydrous metal salts of other anions may be employed provided that the anion is non-acidic and thermally stable and that the metal forms a relatively unstable carbonate when compared to the ori inal carbonate, which it is desired to decompose. As indicated, the temperatures of such reactions may be below the fusion point of such metal salt but are preferably carried out at a temperature within of the fusion point.

It will be seen from the foregoing description that the method of the present invention includes the degradation of a relatively thermally stable carbonate salt by a metathetical reaction to a relatively thermally unstable carbonate salt. This is accomplished simply by heating together in the absence of any other substance the relatively stable metal carbonate salt and an anhydrous salt of a metal and a stable non-acidic inorganic anion, the metal forming a relatively thermally unstable carbonate in the reaction. No added solvent is required in this reaction and indeed the addition of a solvent is objectionable since the solvent will be volatilized on heating and its vapor be carried with the carbon dioxide evolved. The present method is particularly advantageous in that it may be carried out at temperatures ell below the fusion point of glass and thus glass apparatus may be employed. Further, substantially complete reaction is obtained in a relatively short time at a low temperature whereas the calcination of barium carbonate for example would proceed extremely slowly at such low temperatures. The same is true of other thermally stable carbonate salts, such as the carbonates of calcium and strontium. Magnesium and beryllium carbonates are of course quite unstable toward heat. It is necessary that the carbonate salt contain neither chemically nor physically bound water which may be liberated upon heating.

It is notable that the initial reaction, in contrast to a calcination, is a metathesis between a thermally stable carbonate salt and a thermally stable metal salt of a non-acidic inorganic anion. That is to say that during the course of the reaction neither of the original reactants is decomposed into its constituents. Furthermore, since all substances other than the reactants are excluded by the high vacuum and the requirement that no solvent be used, there is no opportunity for either of the reactants to enter into any reaction but the desired metathesis. For example, in the presence of air or moisture many metal halide salts are decomposed at elevated temperatures to yield either the halide or the hydrogen halide. While these acidic gases may assist in the decomposition of the thermally stable carbonate, the additional problem arises of their separation from the product carbon dioxide. The present invention contemplates a new reaction wherein two dry anhydrous salts react directly with each other in a high vacuum to produce by metathesis a thermally stable product and a thermally unstable product, and the latter is decomposed by calcination. For example, the reactions involving barium carbonate and lead chloride may be represented as follows:

A 2. PbCO; PbO+CO Since many embodiments might be made of the present invention and since many changes might be made in the embodiment described, it is to be understood that the foregoing description is to be interpreted as illustrative only and not in a limiting sense.

We claim:

1. In the method of preparing radioactive carbon wherein the carbon is incorporated as carbon dioxide in a carbonate salt of an alkaline earth metal selected from the group consisting of strontium, calcium and barium, the method of liberating the carbon dioxide comprising mixing said carbonate salt in the dry with at least one dry, anhydrous halide salt selected from the group consisting of silver chloride, lead chloride, beryllium chloride, cadmium iodide and copper bromide, heating the mixture at a pressure of less than millimeters of mercury absolute in the absence of any added solvent at a temperature suflicient to liberate carbon dioxide and separating carbon dioxide as a pure, dry gas.

2. The method of claim 1 in which the carbonate salt is barium carbonate.

3. In the method of preparing radioactive carbon in which the carbon is incorporated as carbon dioxide in a carbonate salt of an alkaline earth metal selected from the group consisting of strontium, calcium and barium, the method of liberating carbon dioxide that comprises mixing said carbonate salt in the dry with dry, anhydrous lead chloride and heating the mixture at a pressure of less than 10- millimeters of mercury absolute at a temperature suflicient to liberate carbon dioxide and separating the carbon dioxide as a pure, dry gas.

4. The method of claim 3 in which the mixture is heated at a temperature in the range of 400 to 500 C.

5. In the method of preparing radioactive carbon wherein the carbon is incorporated as carbon dioxide in barium carbonate, the method of liberating the carbon dioxide comprising mixing the barium carbonate in the dry with a dry, anhydrous, eutectic composition of anhydrous lead and silver chlorides and heating the mixture at a pressure of less than 10- millimeters of mercury absolute in the absence of any added solvent at a temperature sufficient to melt said eutectic composition and thereby to liberate the carbon dioxide, and separating the carbon dioxide as a pure, dry gas.

6. In the method of preparing radioactive carbon in which the carbon is incorporated as carbon dioxide in barium carbonate, the method of liberating the carbon dioxide comprising the steps of mixing said barium carbonate in the dry with dry, anhydrous lead chloride, the quantity of lead chloride being from 250 to 500% of the number of moles of said carbonate salt, heating the mixture at a pressure of less than 10- millimeters of mercury absolute at a temperature within C. of the melting point of the lead chloride at a temperature sufiicient to liberate carbon dioxide and recovering the carbon dioxide as a pure, dry gas.

WARREN W. MILLER. JOHN TURKEVICH. NORMAN ZWIEBEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,565,215 Smith Dec. 8, 1925 1,661,043 Koehler Feb. 28, 1928 OTHER REFERENCES J. W. Mellors A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 4, page 507; vol. '7, pages 830, 833, Longmans, Green and 00., N. Y. 

1. IN THE METHOD OF PREPARING RADIOACTIVE CARBON WHEREIN THE CARBON IS INCORPORATED AS CARBON DIOXIDE IN A CARBONATE SALT OF AN ALKALINE EARTH METAL SELECTED FROM THE GROUP CONSISTING OF STRONTIUM, CALCIUM AND BARIUM, THE METHOD OF LIBERATING THE CARBON DIXOIDE COMPRISING MIXING SAID CARBNATE SALT IN THE DRY WITH AT LEAST ONE DRY, ANHYDROUS HALIDE SALT SELECTED FROM THE GROUP CONSISTING OF SILVER CHLORIDE, LEAD CHLORIDE, BERYLLIUM CHLORIDE, CADMIUM IODIDE AND COPPER BROMIDE, HEATING THE MIXTURE AT A PRESSURE OF LESS THAN 10-2 MILLIMETERS OF MERCURY ABSOLUTE IN THE ABSENCE OF ANY ADDED SOLVENT AT A TEMPERATURE SUFFICIENT TO LIBERATE CARBON DIOXIDE AND SEPARATING CARBOM DIOXIDE AS A PURE, DRY GAS. 